418 Transactions. 



of the outgrowth shown in fig. 93. At the tip of another sterile leaf an 

 outgrowth was present alongside the mucro, as if the tip was preparing to 

 fork (figs. 95a and 95b). Such abnormalities may or may not have any 

 significance in indicating that reduction has taken place in these leafy 

 appendages, but they seem to show that the sporophyll and the sterile leaf 

 are not in nature essentially different from one another, and that neither 

 of them is altogether fixed in form. In fact, the sporophyte generation 

 as a whole in Tmesipteris provides many details indicating, as might be 

 expected in a plant showing undoubted primitive characters, a general 

 lack of specialization. 



General Considerations. 



As has been noted above, there is a very striking similarity in form and 

 general external appearance between the prothallus and the young sub- 

 terranean sporophyte of Tmesipteris. It is evident that this similarity 

 between the two generations holds for Psilotum also, judging from Lawson's 

 description (12) of the prothallus and that of Solms Laubach (14) with 

 respect to plantlets developed from buds on old rhizomes. Bearing in mind 

 how plastic, generally speaking, the Pteridophyte gametophyte is known 

 to be, and, moreover, how largely the distribution and persistence of these 

 ancient families has probably been due to the ability of the gametophyte to 

 adapt itself to new conditions, it would be, of course, unwise to conclude too 

 hastily that the similarity between the two generations in the Psilotaceae 

 is a primitive feature. On the other hand, a close correspondence of this 

 nature is not found in the life-history of other modern Pteridophytes, even 

 in those epiphytic forms of Lycopodium and Ophioglossum which possess 

 a cylindrically-built, branched, and ramifying prothallus; nor can this 

 be attributed altogether to the more complex organization of the young 

 sporophyte in these families. Whatever view we take of this external 

 similarity as it is to be seen in the Psilotaceae, the fact that it exists is 

 at least worthy of attention, and becomes even more so when it is found 

 to go hand in hand with certain important structural features in both 

 generations which can with reason be claimed to be primitive. 



The superficial position of the sexual organs on the prothallus of 

 Tmesipteris and Psilotum can be regarded as a structural feature of the 

 gametophyte which has not arisen by modification from a more deeply situ- 

 ated position. The adoption of the subterranean habit of growth in other 

 pteridophytic families has not resulted in a similar simple organization and 

 structure of the sexual organs as is to be found in the Psilotaceae, and it is 

 therefore difficult to see why in the latter this simplicity should be regarded 

 as the result of modification. The persistence of the single apical cell 

 throughout the life of the prothallus, the dichotomous branching, the gradual 

 extension in girth of the prothallus from an initial filament without the 

 formation of such a primary tubercle as is found in some Lycopodiums, and 

 the complete absence of any differentiation of tissues in the prothallus-body, 

 may all be urged as more or less primitive features. 



In correspondence with the superficial position of the sexual organs on 

 the prothallus, the embryo also is shallowly seated, there being no suspensor 

 organ to push it down into the food-supplying tissues of the prothallus 

 such as has been developed in the Lycopodiaceae, and in certain also of the 

 Ophioglossaceae. On the assumption that the suspensor is a primitive 

 organ, it might be urged that it had become lost in Tmesipteris owing to 



